The present invention relates to a golf ball and a die for molding the golf ball.
A golf ball is usually manufactured by coating a core material thereof with a cover layer by using a die. This die is provided with support pins and vent pins. Also, the die includes an upper die and a lower die, which are joined to each other at a parting line. The support pins, vent pins, and parting line produce burrs on the surface of golf ball. The burrs must be removed by grinding, because they may greatly impair the aerodynamic characteristics of golf ball.
If the surface of golf ball, i.e., the surface of cover layer is ground for removing burrs, the dimples formed on the surface of cover layer each must have a specified depth. Therefore, the depths of dimples are measured repeatedly so that the dimples each have a specified depth while the surface of cover layer is ground.
The depth of dimple is usually measured by using a depth micrometer or similar instruments. However, this measuring procedure has a problem that it requires very much time, because the dimple has a shape curved inward. Also, a problem arises in that unless the surface of golf ball is ground properly so that the dimples each have a specified depth, the golf ball does not fly along a desired trajectory.
Japanese Patent Application Publication No. 63-99884 describes a method comprising forming two kinds of depressions having different depths on the surface of golf ball, and then grinding the surface of golf ball until the depressions having a shallower depth disappear. According to this method, a proper grinding quantity can be determined merely by visually observing the depressions.
Japanese Patent Application Publication No. 2003-111868 describes a method comprising forming a depression having three different depths on the surface of golf ball, and then grinding the surface of golf ball. According to this method, the grinding quantity can be readily determined by visually observing the depression, because the number of the steps in the depression which are produced by different depths is changed depending on the grinding quantity.
However, these technique described in the above-mentioned documents may impair the aerodynamic characteristics of golf ball, because deep depressions should be formed on the surface of golf ball. Also, an exact grinding quantity cannot be readily measured by these methods described in the above-mentioned documents, though a rough grinding quantity can be determined by them. Therefore, the exact depth of dimple after grinding cannot be readily measured or controlled by these techniques.